Mr Mwangi gets his son to stand and pull up his white vest to reveal a thick, angry scar covering almost the width and length of his back.
He says the wounds were so deep the surgeon had to remove large pieces of skin from his thighs to use as skin grafts.
"This is him in hospital," says his mother Agnes Mutiri, showing pictures of Caleb on her phone, too graphic to publish. Lying face down on the bed, lacerations cover his legs, back and arms, and even his face. There were almost 100 in total.
"His whole body was like this."
Witnesses say Ebbie Noelle Samuels was beaten because of her hairstyle. [BBC] Reported to TSC
The source said that cases of school beatings reported to the TSC at county level often never go any further, adding that incidents were "killed" and "never see the light of day".
"So many times, by the time the case reaches us, so much evidence has been corrupted. Sometimes we cannot even get a hold of the witnesses."
BBC Africa Eye contacted the TSC to respond to these allegations, but it did not reply.
The thought that a student could die at the hands of education professionals who are supposed to protect them is unimaginable for most people, but in the last five years, more than 20 deaths linked to school beatings have been reported in the media.
Fifteen-year-old Ebbie Noelle Samuels is believed to be one of them.
Ebbie was a boarder at Gatanga CCM Secondary school in Murang'a County.
On March 9, 2019 her mother, Martha Wanjiro Samuels, was called by the school to say her daughter was unwell in hospital.
When she arrived there, Ebbie was already dead.
The school said that she had died in her sleep, but witnesses say she was beaten by the deputy principal because of the way she wore her hair.
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"The autopsy report revealed that she had severe head injury, blunt force trauma. So, somebody hit her to cause that kind of an injury to her, leading to her death," said Ms Samuels. She campaigned for four years to have her daughter's death investigated.
Last January, Elizabeth Wairimu Gatimu, the former deputy principal of Ebbie's school, was arrested for murder. She denies the charges against her.
"I will do everything that I have to do as long as I'm alive to ensure justice is served for my child," said Ms Samuels, who is still waiting to hear the outcome of the case.
"I told myself: 'I will not be silenced. I will not keep quiet. I will not give up fighting.' Maybe the day I give up is the day that I sleep like my daughter. But as long as I breathe, I will not give up."
BBC Africa Eye requested an interview with the Kenyan Ministry of Education, but nobody was willing to speak.
One organisation which is pushing for change is Beacon Teachers Africa. Launched in Kenya four years ago by the non-governmental group Plan International, together with the TSC, its aim is to give teachers the opportunity to protect children in schools and their communities.
It now has a network of 50,000 teachers across 47 countries in Africa.
Robert Omwa is one of 3,000 Beacon teachers in Kenya. As well as educating children about their rights, he also holds workshops to train teachers how to deliver discipline without using corporal punishment.
"Initially, I was sceptical about it. I thought this is Western ideology, an African child has to be beaten. But when I tried it, I felt relieved as a teacher. I felt lighter. I felt the children gravitating more towards me," he said.
Back in Mombasa, Caleb and his family are waiting to hear the fate of his school director. Gachewa has pleaded not guilty.
The 15-year-old still finds it hard to process what happened to him.
"For me to get justice, I want this woman to be jailed."
By Tom Odula in Nairobi & Tamasin Ford in London
BBC Africa Eye